Coupling radiation between two optical chips, e.g. in a stack of optical chips can be done in a variety of ways, such as for example using an optical fiber. For example, radiation propagating in a waveguide in one optical chip, can be transferred to a second optical chip by butt-coupling an optical fiber between the waveguide in the one optical chip and a waveguide in the second optical chip. Nevertheless, coupling between waveguides and an optical fiber typically results in relative high losses and may require accurate alignment.
In order to circumvent these problems, the connection between optical chips is usually performed using further aids, such as tapered intermediate waveguides (which add up new components and more losses) and gratings. Nevertheless, also these connections still suffer from large losses and the need for accurate alignment.
Some conventional mechanisms require no intermediate features such as optical fibers or tapered waveguides or gratings. These mechanisms are based on evanescent wave radiation. When radiation is travelling in a waveguide, the major portion is propagating in the core, but a small portion of the radiation extends outside the core of the waveguide. This radiation, referred to as evanescent radiation, has a strongly decreasing intensity profile with distance from the core. Nevertheless, if a neighboring waveguide is sufficiently close to the first waveguide, evanescent radiation from the first waveguide can be coupled into the neighboring waveguide and in this way coupling of radiation between the two waveguides can be obtained. Such coupling typically is referred to as evanescent wave coupling. Nevertheless, conventional evanescent wave couplers still require accurate alignment in order to guarantee that accurate coupling occurs.